A communication system can be seen as a facility that enables communication between two or more entities such as user equipment and/or other nodes associated with the system. The communication may comprise, for example, communication of voice, data, multimedia and so on.
A communication system typically operates in accordance with a given standard or specification which sets out what the various elements of the system are permitted to do and how that should be achieved. For example, the standard or specification may define if the user, or more precisely, user equipment or terminal is provided with a circuit switched service and/or a packet switched service. Communication protocols and/or parameters which shall be used for the connection may also be defined. In other words, a specific set of “rules” on which the communication can be based on needs to be defined to enable communication by means of the system.
Communication systems providing wireless communication for user terminals or other nodes are known. An example of the wireless systems is a cellular network. In cellular systems, a base transceiver station (BTS) or similar access entity serves mobile stations (MS) or similar user equipment (UE) via a wireless interface between these entities. The operation of the apparatus required for the communication can be controlled by one or several control entities. The various control entities may be interconnected. One or more gateway nodes may also be provided for connecting the cellular network to other networks, such as to another cellular system or to a public switched telephone network (PSTN) and/or other communication networks such as an IP (Internet Protocol) and/or other packet switched networks. The communication between the user equipment and the elements of the communication network can be based on an appropriate communication protocol such as the session initiation protocol (SIP).
For example, in the current third generation (3G) multimedia network architectures it is assumed that various servers are used for handling different functions. These include functions such as call state control functions (CSCFs). A call state control function entity may provide functions such as proxy call state control (P-CSCF), interrogating call state control (I-CSCF), and serving call state control (S-CSCF). The serving call state control function can be divided further between originating call state control function (O-CSCF) and terminating call state control function (T-CSCF) at the originating and terminating ends of a session, respectively. Control functions may also be provided by entities such as a home subscriber server (HSS) and various application servers.
From the above mentioned control entities the home subscriber server (HSS) is for storing subscriber related information. The subscriber information may include data such as registration identities (ID) of the subscriber or the terminals and so on. The home subscriber server (HSS) can be queried by other function entities, e.g. during registration and session set-up procedures. It shall be appreciated that the term “session” refers to any communication a user may have such as to a call, data (e.g. web browsing) or multimedia communication and so on.
A user in communication with an access entity of the communication system may be registered in a serving controller entity such as in a S-CSCF. During registration proceedings various authentication queries or messages and authentication parameters such as those based on authentication quintets and/or keys may be transferred between the entities involved in the process. A user may have more than one registration at the same time. The registration is required for communication via the communication system.
During a registration procedure a user identifier such as a public ID is provided for the controller entity. Public ID is an identifier of a user of the communication system and the basic function thereof corresponds a telephone number of the known telephone systems. A 3G phone may have more than one identifier (e.g. a telephone number, one or more email-like addresses, for example session initiation protocol (SIP) universal resource locators (URL), an address or addresses for alternative communications means, and so on).
Once a public ID is registered it is stored both in a P-CSCF and S-CSCF. An expiration timer is associated with the stored public ID. The timer functions such that when the expiration timer expires the registration of that specific public ID becomes invalid.
In the IP multimedia subsystem IMS currently proposed by the third generation standards, the subscriber is authenticated at registration and can be re-authenticated if the subscriber re-registers. However, the inventors have appreciated that from time to time, events occur in the network which require the re-registration and/or re-authentication of the subscriber. For example:                the length of time for which a subscriber can be registered to the network has a time limit. If the subscriber does not re-register in time, the subscriber becomes unregistered. This can have the result, for example that ongoing calls must be dropped and the subscriber becomes unavailable;        the network becomes distrustful of the subscriber (for example the subscriber is making a very long call) and would like to re-authenticate the user during the call; and        there is an error condition in the S-CSCF (for example congestion, overloading or the like etc.) and S-CSCF would like to force the user to use another S-CSCF.        
It has been suggested that the user should send periodic re-registration messages to the network. This allows re-authentication also to be performed. This does not deal with the problems which arise when the network is suspicious of a user. The network has to wait for the next re-registration message. Again if a change in the S-CSCF is required, again the network must wait for the next re-registration message.
If the re-registration messages are too frequent, then network resources may be used unnecessarily.
It has also been proposed that the session initiation protocol SIP allow for the authentication or re-authentication of a subscriber when he/she sends a new SIP request (e.g., INVITE). However forcing authentication when an INVITE has been received would unacceptably increase the session set-up time.
The subscriber can be de-registered. A notification is sent to the subscriber about this if there is a subscription for the notification. On receipt of this notification, the user can re-register. This has the disadvantage that any ongoing calls will be dropped.
It shall be appreciated that although the above discussed the registration proceedings and related problems with reference to an internet protocol (IP) based third generation (3G) communication system and session initiation protocol (SIP), similar disadvantages may associate with other systems as well and thus the description is not limited to these examples.